What's Up Doc? Long and short of kids' height

Tuesday

There are many reasons a child may be shorter than their peers. We first need to decide what ``short'' means, since kids will, of course, come in many shapes and sizes.

Q: My neighbor's daughter is in the fifth grade with my son, but she is as small as my daughter who is in first grade. Her parents are average height. My neighbor told me her daughter is on medication now. What can this be from?

A: There are many reasons a child may be shorter than their peers. To discuss this we first need to decide what ``short'' means, since kids will, of course, come in many shapes and sizes.

Most everyone is familiar with the growth charts pediatricians use to monitor the growth and development of children. These charts represent measurements of large numbers of children noting what average (mean) height and weight are by age. For those of you who hate math, skip over my more detailed discussion of this in the next paragraph.

When you plot the number of children of a certain age with a specific height or weight versus what that height or weight is you get a ``normal'' distribution; that is, one of those bell-shaped curves we all had to learn about in school. The standard deviation is a number that can be calculated, which measures how flat or pointy the distribution is. Two-thirds of a normal distribution lies within one standard deviation of the average, and 95 percent lies within two standard deviations. For example, the average height for 5-year-old girls is about 42 inches, with 95 percent of them being between 39 and 45 inches. So in this example, the mean height is 42 inches, with a standard deviation of 1-1/2 inches.

Understanding this we can now define what we mean by being ``short.'' If a child is not within two standard deviations of the average height for their age (that is, they are in the shortest 2.5 percent of kids their age, since the other 2.5 percent not within two standard deviations will be the tallest), they are considered to have short stature.
Kids with short stature are usually placed into three different categories: familial short stature, constitutional delay and growth disorders.

Familial short stature (sometimes called intrinsic shortness) is the category where kids are short because they have inherited a tendency to be short (they have short relatives, but their genetic makeup is normal). These kids will grow along (or parallel to) the lines in the growth chart, but at the very short end of the height spectrum. If X-rays are done to look at the bones of these children, it is found that their ``bone age'' (how developed the bones are compared to other children of the same age) is consistent with their age in years. Although there is no way to predict for sure how tall any child may be, one way to estimate a child's eventual adult height is to estimate it based on the height of their parents as follows:

Note that this gives only a very rough estimate of what a child's adult height will be, but it is one piece of evidence to help decide if a child has familial short stature.

In summary, these kids have no genetic abnormalities and no medical conditions causing their short stature, and they achieve their ``genetic potential'' even though that is shorter than their peers.

Constitutional delay (sometimes called delayed growth) is very different from familial short stature. These kids are shorter than their peers because they have not had the growth spurts their peers have had. If X-rays are done to look at the bones of these children, their bone age will be younger than their age in years. These kids will eventually catch up when they have their delayed growth spurt and become similar in height (on average) to their peers; in lay terms we call these kids late bloomers.
In summary, these kids have no genetic abnormalities and no medical conditions causing their short stature, and they achieve their genetic potential height, just delayed compared with their peers.

The last category is growth disorders (sometimes called attenuated growth). These kids have some medical reason for being shorter than their peers. There are certain genetic syndromes that can cause this (for example, Turners syndrome, where a young girl has only one X chromosome instead of two). There are also many medical conditions that can occur in children with normal genetic makeup that can cause this. For example, kidney failure, liver failure and some hormone conditions (such as growth hormone deficiency, thyroid deficiency or Cushing syndrome) can cause short stature. Certain heart and lung conditions can use up the nutritional energy a child needs to grow. Certain digestive conditions or other causes of malnutrition (such as certain infections or insufficient available food) can prevent the child from getting adequate nutritional energy to grow. Worldwide, inadequate nutrition is the leading cause of short stature.

Some of the causes of growth disorders are treatable, so it is this category that is most important to recognize. Furthermore, the earlier treatment is begun for these treatable conditions, the better the chance the child will achieve their genetic height potential. For example, it is best to treat a low thyroid condition at least five years before the child enters puberty in order to achieve the best results.

The question now becomes: How common are treatable causes of short stature? A study done in Utah in 1994 has some of the best data to address this issue. Over 115,000 kids were analyzed, and 555 (0.7 percent) were found to be both under the 3 percent mark in height and also to have growth of less than 5 centimeters per year (the lower limit of what is considered normal).

Causes of short stature in these kids were found to be: familial in 37 percent, constitutional delay in 27 percent, a combination of familial and growth delay in 17 percent, and growth failure in only 22 percent (round-off errors explain why this adds up to more than 100 percent). Of the 22 percent that were due to growth failure: 10 percent were due to ``other medical causes,'' 3 percent to growth hormone deficiency, 0.5 percent to thyroid problems, 3 percent due to Turner's syndrome and 5 percent idiopathic (unknown causes).

I hope this discussion reinforces for everyone the importance of those well-child checkups with your health care provider. Noting a possible problem early, when a child just begins to ``fall off'' the growth curve, will allow earlier identification of a possible issue and hence earlier initiation of treatment. So keep those well-child appointments. They are very worthwhile.